In mobile communication, compression coding for digital information about speech and images is essential for an efficient use of transmission bands. Especially, speech codec (encoding and decoding) techniques widely used in mobile phones are highly expected, and there is an increasing demand for further improved sound quality in conventional high-efficiency coding with high compression performance.
Recently, with broadbandization of communication networks, there is a demand for realization and high sound quality in speech communication, and, to meet this demand, speech communication systems using stereo speech coding techniques have been developed.
As a method of encoding stereo speech, there is a known conventional method of finding a monaural signal and side signal and encoding these signals, where the monaural signal is a sum of the left channel signal and the right channel signal and where the side signal is the difference between the left channel signal and the right channel signal (see Patent Document 1).
The left channel signal and the right channel signal represent sound heard by human's left and right ears, the monaural signal can represent the common elements between the left channel signal and the right channel signal, and the side signal can represent the spatial difference between the left channel signal and the right channel signal.
There is a high correlation between the left channel signal and the right channel signal. Consequently, compared to a case where the right channel signal and the left channel signal are encoded directly, it is possible to perform more suitable coding in accordance with the features of the monaural signal and the side signal by converting the right channel signal and the left channel signal into a monaural signal and side signal and then encoding these converted signals, so that it is possible to realize coding with less redundancy, low bit rate and high quality.
Recently, standardization of scalable codec having a multilayer configuration is studied in, for example, ITU-T (International Telecommunication Union Telecommunication Standardization Sector) and MPEG (Moving Picture Expert Group), and more efficient and higher-quality speech codec is demanded.
For example, a scalable coding apparatus based on ITU-T G.729.1 performs ITU-T recommendation G.729.1 coding of 8 kbps, and, by further encoding an enhancement layer, can perform coding of twelve kinds of bit rates such as 8 kbps, 12 kbps, 14 kbps, 16 kbps, 18 kbps, 20 kbps, 22 kbps, 24 kbps, 26 kbps, 28 kbps, 30 kbps and 32 kbps. This scalability is realized by sequentially encoding lower layer coding distortion in higher layer. That is, the G.729.1 scalable coding apparatus is formed with one core layer of a bit rate of 8 kbps, one enhancement layer of a bit rate of 4 kbps and ten enhancement layers of a bit rate of 2 kbps.
Also, as a technique of performing scalable coding of stereo signals, there is a stereo signal coding apparatus disclosed in Patent Document 2. This stereo signal coding apparatus expresses additional information for each layer by a predetermined number of bits, and, using a predetermined probability model, performs arithmetic coding of bit sequences in order from the most significant bit sequence to the least significant bit sequence. Here, this stereo signal coding apparatus has a feature of switching between the left channel signal and the right channel signal according to a predetermined rule and encoding these signals.    Patent Document 1: Japanese Patent Application Laid-open Number 2001-255892    Patent Document 2: Japanese Patent Application Laid-open Number HEI 11-317672